Electronic devices that are used to detect gases are sometimes referred to as “electronic noses” or “artificial noses.” Possible uses include detecting a presence of an unhealthy environment and identifying constituents of a liquid or gas. Major goals in the design of gas-detection devices include minimizing costs and maximizing reliability and speed.
There are a number of different approaches to detecting and identifying gases. One approach is to employ conductive transducers that change electrically when particular gases are introduced. The electrical change may be with respect to resistance or capacitance. The transducers may be an array of metal oxide pads or chemically absorbent pads which have different specific reactions to gases (i.e., analytes). With properly designed arrays, each of a number of different gases will have a unique characteristic set of resistance/capacitance values when the array of transducers is exposed to the gas.
A second general-category approach to designing an electronic nose is to include absorbent polymers in a quartz crystal microbalance (QCM) system. The absorbent polymers will have masses that vary as different molecules are absorbed. As a result, the resonant frequency of the system will change in dependence upon the molecules to which the polymers are exposed. The third approach is similar, since frequency changes are used to identify gases. In this third approach, a surface acoustic wave (SAW) system is involved, with the frequency variations being with respect to travel along a surface, rather than through a bulk material.
There are at least two optical approaches. In one such approach, the electronic nose includes an array of transducers which are chemically active fluorescent dyes. As analytes interact with the fluorescent dyes, light is generated by the various dyes. The frequencies of the emitted lights are used to identify the gas or gas components. The other optical approach is to utilize dyes which merely change spectral characteristics (color) as a reaction to exposure to fluid molecules. This approach is described in detail in U.S. Pat. Nos. 6,495,102 and 6,368,558 to Suslick et al. By selecting the proper array of dyes, gases can be distinguished on the basis of distinct spectral responses of the array. As one possibility, the dyes may be metalloprophyrins. Referring to FIG. 1, an array of metalloprophyrins 10, 12,14, 16, 18 and 20 is formed on a plate 22. The Suslick et al. patents state that the plate may be formed of a number of different materials, including paper, porous membranes, polymers, glasses or metals. A light source 24 is used to illuminate the elements of the array. An imager 26 may be a charge coupled device (CCD), but the patents state that a flatbed scanner may be employed for the imaging tasks. While not shown in FIG. 1, one or more optical components may be used between the array and the imager. The output of the imager is directed to a data processing unit 28. In use, the array is exposed to an analyte and the imager 26 detects the resultant color pattern. The data processing unit 28 is able to identify at least one component of the analyte on the basis of the imaged color pattern. The process operates well for its intended purposes, but advances are available.